Light emitting diode (LED) lighting has become very popular in the industry due to their many advantages including a longer lifespan, fewer hazards and increased visual appeal when compared to other lighting technologies, such as for example compact fluorescent lamp (CFL) or incandescent lighting technologies. The advantages provided by LED lighting have resulted in LEDs being incorporated into a variety of lighting technologies, televisions, monitors and other applications that may also require dimming.
One known technique for dimming is the use of a triac or phase angle dimming. A triac circuit operates by removing some beginning or ending portion of each half-cycle of ac power, which is known as “leading edge or trailing edge phase control” respectively. By eliminating some portion of each half-cycle, the amount of power delivered to the lamp is reduced and the light output appears dimmed to the human eye. In most applications, the missing portion of each half-cycle is not noticeable to the human eye because the variations in the phase controlled line voltage and the variations of power delivered to the lamp occur so quickly. While the triac dimming circuits work especially well to dim incandescent light bulbs when they are used for dimming LED lamps they are likely to produce non-ideal results, such as flickering, blinking, color shifting and input waveform distortions.
A difficulty in using triac dimming circuits with LED lamps comes from a characteristic of the Triac itself. A Triac behaves as a controlled ac switch that is open until it receives a trigger signal at a control terminal, which causes the switch to close. The switch remains closed as long as the current through the switch is above a value referred to as the holding current. Most incandescent lamps easily draw more than the minimum holding current from the ac power source to enable reliable and consistent operation of a triac. However, the LEDs low currents may not be enough compared to the minimum holding currents required to keep Triac switches conducting for a reliable operation.
Even though the triac-based, leading or trailing edge phase control dimmer LED drivers may provide light dimming with a low cost, since the current drawn by the string/array of LED lamps may easily go below the holding current of the triac dimming circuit, it may cause the undesirable behavior including limited dimming range, flickering and input waveform distortions. The Triac may fire inconsistently as a result of the low current drawn by the LED string/array. In addition, due to the inrush current charging the input capacitance and because of the relatively large impedance that the string/array of LEDs presents to the line, a significant ringing may occur whenever the Triac turns on. This ringing may cause even more undesirable behavior as the Triac current could fall to zero and turn off the string/array of LEDs, resulting in flicker.
As a consequence, the LED driver may include a compensator circuit (e.g., a bleeder, or dummy load/preload) to increase current demand from the Triac dimmer. A bleeder circuit is in fact a dummy load of passive or active components added at the input or output to take enough extra current in addition to the LEDs to keep the Triac conducting reliably after it is triggered. Low cost passive bleeders are common and popular in LED driver circuits but have associated penalties in efficiency and performance.